In recent years it's started a bit earlier. We checked in last Thanksgiving on the Tuesday before Thanksgiving, and the resort was already decorated. I'm guessing that they may have made too many cutbacks to do everything in one night.
We checked in last year on the Saturday before Thanksgiving and the majority of the decorations were up then. We did see a few additions as the week progressed though. I think perhaps Disney is trying to let the Thanksgiving crowd enjoy the decorations too--so they are putting things up earlier than they used to.